Conventionally, there is known a plasma generation device that generates electromagnetic wave plasma by emitting an electromagnetic wave to a combustion chamber of an engine. For example, Japanese Unexamined Patent Application, Publication No. 2009-221948 discloses a technique of generating microwave plasma by emitting a microwave from an antenna, while causing a discharge at electrodes of a discharger in a combustion chamber of an engine.
Furthermore, as a method of impedance matching for frequency of microwave band, an open circuit or short circuit stub is employed. Japanese Unexamined Patent Application, Publication No. 2004-7248 and Japanese Unexamined Patent Application, Publication No. 1995-153599 disclose methods of mechanically adjusting insert amount of a stub as a method of stub adjustment in accordance with load variation. Furthermore, Japanese Unexamined Patent Application, Publication No. 2007-174064 and Japanese Unexamined Patent Application, Publication No. 2009-268004 disclose an adjustment unit of the open circuit stub.